1. Field of the Invention
The invention relates to a gear that is formed by coupling a synthetic resin tooth body to the outer peripheral portion of a metal core, and an electric power steering device that includes the gear.
2. Description of the Related Art
Japanese Patent Application Publication No. 2001-206230 (JP-A-2001-206230) describes an electric power steering device for a vehicle, in which steering torque applied to an input shaft connected to a steering wheel is detected based on the relative rotation of the input shaft and an output shaft that is coaxially connected to the input shaft via a torsion bar, and an electric motor for steering assistance is driven based on, for example, the detected torque. The rotating force of the electric motor is transmitted to steering means via a gear pair. Thus, the operation of the steering means in accordance with the rotation of the steering wheel is assisted using the rotation of the electric motor, and therefore, the steering effort of a driver is reduced.
In the electric power steering device in which the gear pair is used, a gear that includes a synthetic resin tooth body is used to reduce noise caused by engagement of the gear pair.
The gear includes a metal core, and the synthetic resin tooth body that is coupled to the outer peripheral portion of the metal core (refer to JP-A-2001-206230). The metal core may be formed by cutting process, cold forging, or press forming. In the case where the metal core is formed by cutting process or cold forging, a plurality of rotation prevention grooves is formed on the outer peripheral portion of the metal core as shown in FIG. 11 and FIG. 12 of JP-A-2001-206230. In the case where the metal core is formed by press forming, a plurality of rotation prevention grooves and a plurality of through-holes are formed on the outer peripheral portion of the metal core as shown in FIG. 7 of JP-A-2001-206230.
The metal core formed in the above-described manner is placed in an injection mold, and integrally coupled to the synthetic resin tooth body by injection molding. The rotation prevention grooves, or the rotation prevention grooves and the through-holes prevent the relative rotation of the synthetic resin tooth body and the metal core.
However, in the gear that includes the metal core formed by cutting process or cold forging, because the relative rotation of the synthetic resin tooth body and the metal core is prevented only by the rotation prevention grooves formed on the outer peripheral portion of the metal core, the rotation prevention grooves are not sufficiently long, that is, the area where the metal core and the synthetic resin tooth body are coupled to each other is not sufficiently large. Thus, it is desired to increase the coupling strength between the metal core and the synthetic resin tooth body in a rotational direction. In the gear that includes the metal core formed by press forming, because the relative rotation of the synthetic resin tooth body and the metal core is prevented by two means, that is, the rotation prevention grooves and the through-holes formed on the outer peripheral portion of the metal core, the area where the metal core and the synthetic resin tooth body are coupled to each other is sufficiently large. However, the structure for preventing the relative rotation is complicated, and therefore, the structure of a die used to form the metal core is also complicated. Thus, further improvement is required.